Download Phylum Platyhelminthes - University of Evansville

Phylum Platyhelminthes
General Characteristics
• They exhibit bilateral symmetry:
anterior and posterior ends are different;
so are the dorsal (top) and ventral
(bottom) surfaces
• The platyhelminths also exhibit some
degree of cephalization Commonly
referred to as the 'flatworms' because
their bodies are dorsoventrally flattened.
• They are acoelomates
• This phylum (and all remaining phyla)
possess 3 germ layers (=triploblastic)
• The mesoderm (third germ layer) gives
rise to muscles, various organ systems,
and the parenchyma, a form of solid
tissue containing cells and fibers
Outer Body Covering
• The body of some platyhelminthes
(e.g., turbellarians) is covered by a
ciliated epidermis
• Epidermal cells contain rodshaped structures called rhabdites
that when released into the
surrounding water, expand and form
a protective mucous coat around the
animal
• The outer body covering of other
platyhelminthes (e.g., parasitic
forms) is a non-ciliated tegument
• The tegument is referred to as a
syncytial epithelium
Organ Systems of the Platyhelminthes
Digestive System
• Some of the flatworms possess a digestive system, with a mouth,
pharynx, and a branching intestine from which the nutrients are
absorbed
• The intestine, with only one opening, is a blind system
Organ Systems of the Platyhelminthes cont.
Excretory System (osmoregulation)
• A network of water collecting tubules adjacent to flame cells
or a protonephridia
• When cilia beat they move water into the tubules and out the
body through pores called nephridiopores
Organ Systems of the Platyhelminthes con’t
Muscular System
• Below the epidermis are layers of
circular and longitudinal muscle
fibers; used in locomotion
Nervous System
• Includes: anterior cerebral ganglia,
longitudinal nerve cords, and some
lateral nerves
• Most free living planarians and
parasitic larval forms possess a
variety of sensory organs (e.g., eye
spots, statocysts, rheoreceptors)
Organ Systems of the Platyhelminthes cont.
Reproductive System
• Most are capable of some
form of asexual
reproduction (e.g., many
turbellarians reproduce by
fission)
• Most flatworms are
hermaphroditic; however,
they often pair with other
individuals to exchange
gametes
Platyhelminthes Taxonomy
Class Turbellaria
• Free-living flatworms; mostly marine organisms
• Range in size from microscopic (interstitial species between
sand grains) to extremely large (two feet)
Locomotion
• Most move by means of
cilia and mucous
• Muscle contractions also
permit turning, twisting and
folding of the body
Class Turbellaria con’t
Nutrition
• Turbellarians are carnivores and prey on other animals or eat dead
animal remains.
• Planarians have a muscular pharynx that they can insert into their
prey and then pump to bring in food fragments
• These animals have a highly divided gut to greatly increase the
surface area for digestion and absorption
Senses
• They have well developed sensory structures, including eyespots,
mechanoreceptors, and chemoreceptors
Class Turbellaria con’t
Reproduction
• Planarians are capable of asexual
reproduction via fission
• Also capable of regeneration; exhibit
both anterior- posterior and lateral
polarity
• They are hermaphrodites but usually
exhibit cross-fertilization
• The penis of some turbellarians is
modified as a hollow stylet; sperm
tranfer is by hypodermic
impregnation, in which the
copulating partners stab each other
and inject sperm
Class Trematoda
• Flukes that live as parasites either on or in other organisms.
• Outer body lacks cilia; tegument has a layer of glycoproteins that are
important in protection and absorption
• Possess 2 suckers:
1. Oral sucker which attaches to organs of the host
2. Ventral sucker or acetabulum; used to attach to host tissues
Types of Hosts
• Often have complex life cycles that alternate between sexual and
asexual stages.
• Most require at least 2 different kinds of hosts to complete their life
cycle:
1. Definitive host (primary host)
• The host in which the parasite matures and reproduces (sexually)
• The host in which eggs are released
2. Intermediate host
• Hosts in which larval stages develop and undergo asexual
reproduction
• Results in an increase in the number of the individuals
General Life Cycle - Chinese liver fluke, Clonorchis sinensis
• Adults live in the bile ducts of humans, dogs, and cats
• There are 2 intermediate hosts: a snail and a fish
• Eggs are passed out of the definitive host and hatch as ciliated larvae
called miracidia
• The miracidia penetrates a snail molluscan host and becomes a
sporocyst
• They undergo asexual reproduction producing larvae called rediae
• Rediae often asexually produce more rediae, but will eventually give
rise to larvae called cercariae
• They leave the molluscan host and penetrate fish
• They encyst in the fish tissues as the metacercaria
• Consumption of infected fish results in the metacercaria excysting in
the gut and migrating to the bile duct
Schistosoma
• Schistosoma spp. is a common
blood fluke of Southeast Asia that
causes shistosomiasis
• Humans are the definitive host;
snails are the intermediate host
• In humans its eggs ultimately
penetrates and damages intestinal
tissue and tissue of the bladder
• A source of constant
inflammation and eventually
leads to deterioration of liver,
spleen and other organs
Class Cestoda
General Morphology
• Nonciliated tegument composed of glycoprotein
• The anterior region is called a scolex; often armed with suckers
and hooks
• Extending from the neck is a
series of proglottids; contain the
sex organs and eggs; no digestive
system
• Mature eggs released through
an opening in the proglottid or
leave the host when the
proglottids are separated from
the main body of the worm.
Beef Tapeworm, Taeniarhynchus saginatus
• Definitive host humans; intermediate host cattle
• Eggs are shed with human feces; infected persons defecate in a
pasture and the eggs are ingested by cattle
• Eggs hatch giving rise to oncosphere larvae that bore into the
intestinal wall and get into the circulatory system to be transported to
muscle
• Here the larvae develop into the cysticercus stage (=the bladder
worm) with the inverted scolex
• If uncooked beef is consumed the cysticercus is freed and the
scolex everts, forming the adult
• Symptoms include loss of weight, chronic indigestion, diarrhea